1. Field of the Invention
This invention relates to subsea structures, and more particularly, to apparatus for protecting subsea wells from damage by trawl boards and ship anchors.
2. Description of the Prior Art
The production of oil and gas from offshore wells has developed into a major endeavor of the petroleum industry. Wells are commonly drilled throughout the world at a wide variety of locations, some of which are in shipping lanes or areas of high density ship traffic while others are in fishing areas where fishing vessels may pull fishing nets, trawl boards and anchors across wellheads having christmas trees attached thereto. A snagged anchor can literally tear a christmas tree off a wellhead thereby causing dangerous and expensive oil and gas leakage into the sea. Such leaks are not only expensive in terms of repair expense and wasted products but can cause considerable damage to the environment. As a result some governments require protective enclosures of subsea production systems, oil storage areas, manifolds, and christmas trees and other wellhead equipment.
Some of the prior art protective enclosures include installations of concrete, steel and/or fiberglass structures that are simply placed over the christmas tree. These structures may be in the form of domes, pyramids or cone-like shapes that allow the deflection of trawl boards and other objects over the christmas trees.
Most steel and concrete protectors rely on their massive weight to retain them in position when they are positioned on the sea floor. Since the christmas tree of a well can protrude 20 to 30 feet above the sea floor, a protective structure such as a steel pyramid may be as tall as 30 feet with a base as large as 50 to 60 feet in diameter, and with a total weight approaching or exceeding 200,000 lbs. The lighter fiberglass structures require some means of anchoring the structures to the sea floor. In some of the prior art installations, this has been accomplished with screw-type anchors positioned around the fiberglass structure. Furthermore, the physical size of all these structures makes their handling and installation difficult.
It is important that the problems of handling the large protective structures and of securing them in position around the subsea christmas trees be reduced. A structure which covers the christmas tree and relies upon its own weight to prevent displacement by moving trawl board or anchors is too large and cumbersome to be conveniently installed.